Make linkpred easier to use from within Python

[rafguns]

The prediction part is fairly straightforward, howver the evaluation part is terribly convoluted. Much of the heavy lifting is done by the LinkPred object. I can think of two steps:

1. Make LinkPred easier to use. For instance:
   • It should be possible to just call LinkPred() without any arguments.
   • It supposes that the training and test networks still have to be read from a file, which is unlikely if used within Python
   • Preprocessing is hard to control exactly.
2. Simplify the whole evaluation part. See #3.